Artificial Leather and Production Method Therefor

ABSTRACT

Provided is an artificial leather that has texture (stiffness), a luxuriant feel (dispersibility of fiber bundles), and a slick feel (resin clusters of appropriate size) and can be used suitably as a seat cover material or interior design material for interiors, cars, airplanes, railway cars, etc. and garment accessory products. This artificial leather comprises a fiber sheet and a polyurethane resin and is characterized in that: the fiber sheet comprises at least a fiber layer (A) that forms a first outer surface of the artificial leather; the k-nearest neighbor distance proportion (k=9, radius r=20 μm) between cross-sections of single fibers configuring the fiber layer (A) in a thickness direction cross-section of the artificial leather is 10-80%; the cross-sectional polyurethane resin area ratio in a thickness direction cross-section of the fiber layer (A) is 10-30%; and the standard deviation of the cross-sectional polyurethane resin area ratio in a thickness direction cross-section of the fiber layer (A) is 25% or less.

FIELD

The present invention provides artificial leather that has texture (stiffness), a luxuriant feel (dispersibility of fiber bundles) and a slick feel (resin masses of appropriate size), as well as a method for producing it.

BACKGROUND

Artificial leather that has a fiber sheet such as a nonwoven fabric (fibrous base material) and a polyurethane (PU) resin as the major materials exhibits excellent properties that are difficult to achieve with natural leather, including easy care, functionality and homogeneity, and it is therefore suitable for use in clothing, shoes and bags, as well as for sheet covering materials and interior finishing materials for interiors, automobiles, aircraft and railway vehicles, or decorative materials such as ribbons or patch bases.

One commonly used method for producing such artificial leather in the prior art is a method of impregnating a fiber sheet with an organic solvent solution of a PU resin, and then immersing it in a non-solvent for the PU resin (such as water or an organic solvent), for wet coagulation of the PU resin. PTL 1, for example, uses an organic solvent-based PU resin with N,N-dimethylformamide as the organic solvent for the PU resin. However, because common organic solvents are very harmful for the human body and the environment, there is a strong demand for methods of producing artificial leather that do not use organic solvents.

PTL 2 examines a method of using a water-dispersed PU resin dispersion with PU resin dispersed in water, as a substitute for conventional organic solvent-based PU resins, but sheets obtained by impregnating fiber sheets with water-dispersed PU resin dispersions to coagulate the PU resins tend to exhibit a hard texture. One major reason for this is the difference in the coagulating systems of the two types. Specifically, the coagulating system of an organic solvent-based PU resin dispersion is a “wet coagulation system” wherein an organic solvent dissolving the PU molecules is solvent-exchanged with water to precipitate out the PU molecules, and in the case of a PU film it forms a porous film of low density. Even when a PU resin has been impregnated into a fiber sheet and coagulated, therefore, punctiform adhesion points between the fibers and the PU resin exists, and since the PU resin tends to have a porous structure this results in a soft sheet. A water-dispersed PU resin, on the other hand, is a “dry heat coagulating system” wherein the hydrated state of the PU molecules dispersed in water is disintegrated primarily by heating, and the PU molecules are caused to aggregate together causing coagulation, resulting in a non-porous film with a highly dense PU film structure. Adhesion between the fibers and PU resin is therefore dense and the intertwined portions of the fibers are firmly held fast, resulting in a hard texture. In light of proposed techniques for forming a porous structure of PU resins in sheets in order to improve texture by the use of a water-dispersed PU resin, or in other words, to prevent the PU resin from tightly holding the entangled points of the fibers, it is disclosed that by implementing a method for producing a sheet in which a PU resin dispersion comprising a water-dispersed PU resin, a foaming agent and an anionic surfactant and/or amphoteric surfactant is added to a fiber sheet, it is possible to create a porous structure of the PU resin inside the sheet regardless of the type of foaming agent or PU, and it is possible to produce a sheet having uniform nap lengths similar to artificial leather types that use organic solvent-based PU resins, while also having a delicate surface quality with an excellent luxuriant fiber feel and a satisfactory flexible texture with excellent repulsive properties as well.

However, a sheet obtained by the method described in PTL 2 has large voids between the ultrafine fiber bundles and the PU resin (the PU resin has a porous structure), so that firm bonding of the PU resin to the ultrafine fiber bundles is inhibited resulting in a partially flexible texture, but the cross-sectional area ratio of the PU resin is still relatively high and the dispersibility of the PU resin is not sufficient, while the dispersibility of the single fibers is not considered in this publication.

PTL 3 discloses a sheet having a uniform feel comparable to artificial leather that uses an organic solvent-based PU resin, and a delicate surface quality and satisfactory texture, as well as a method for producing it, with the goal of providing a sheet having a PU resin porous structure by the use of a water-dispersed PU resin and having wrinkle recoverability and flexibility very similar to artificial leather that uses an organic solvent-based PU resin, as well as a method for producing the same, where the solution means disclosed is a sheet that is composed of a fiber sheet comprising ultrafine fibers and/or ultrafine fiber bundles, with a elastomer having hydrophilic groups (such as a water-dispersed PU resin) added as a binder, and wherein in a cross-section of the sheet cut in the thickness direction, the proportion of independent sections with a cross-sectional area of 50 μm² or greater in the elastomer observed within the cut surface is 0.1% to 5.0% of the cross-sectional area of the artificial leather in the observation field, the method proposed by this publication for producing it being a sheet-forming method wherein a elastomer having a hydrophilic group is added as a binder to a fiber sheet composed of ultrafine fibers and a dispersion containing the elastomer and a thickener is applied to the fiber sheet, after which the elastomer is coagulated in hot water at a temperature of 50 to 100° C. In the same publication it is stated that the thickener used is guar gum which has high thixotropy at low concentration, that the thixotropic dispersion results in lower viscosity when force is applied by stirring or the like, allowing the dispersion to be uniformly impregnated into the fiber sheet, and that the viscosity is restored if the dispersion is left to stand after application of force, such that the dispersion impregnated into the fiber sheet is less likely to be shed from the fiber sheet.

With a sheet obtained by the method described in PTL 3, however, even though control of the size of the water-dispersed PU resin (to small PU resin mass sizes) results in some improvement in resin loss, texture and appearance quality, the area ratio of the cross-sectional PU resin is high and dispersibility of the PU resin is not sufficient, while the dispersibility of the single fibers is also not considered in this publication.

In PTL 4 there is proposed an artificial leather base material comprising a three-dimensional intertwined nonwoven fabric and a elastomer, for the purpose of providing an artificial leather base material exhibiting excellent mechanical properties, flexibility, texture and lightweight properties, wherein the artificial leather base material satisfies the following condition (1): the elastomer on the surfaces of the fibers forming the three-dimensional intertwined nonwoven fabric is discontinuous; condition (2): the average area of inter-void inscribed circles excluding inter-void inscribed circle areas of less than 350 μm² in parallel cross-sections in the thickness direction of the artificial leather base material is 1250 μm² or lower; and condition (3): the number of inter-void inscribed circles with inter-void inscribed circle areas of 350 to 3000 μm² in parallel cross-sections in the thickness direction of the artificial leather base material is at least 85% with respect to the total number of inter-void inscribed circles. This type of artificial leather base material can be obtained by a process of impregnating the three-dimensional intertwined nonwoven fabric with a water-dispersed PU resin dispersion having polyvinyl alcohol (PVA) resin added, and coagulating it to form a elastomer. According to PTL 4, since the elastomer has a consistent non-contact structure with the fibers and is discontinuous on the fiber surfaces, with the PU resin evenly dispersed in the interior, the voids are evenly distributed between the fibers to provide an artificial leather base material exhibiting excellent mechanical properties, flexibility and lightweightness suitable for use in sports shoes, for example, and having an excellent textural feel.

However, although the artificial leather base material obtained by the method described in PTL 4 has improved dispersibility of the water-dispersed PU resin and exhibits flexibility and light weight, the dispersibility of the single fibers is less than satisfactory and it therefore lacks a luxuriant feel. The artificial leather base material described in PTL 4 is also primarily used for “smooth” artificial leather that has a grain side appearance.

PTL 5 proposes a sheet comprising a elastomer inside a fiber sheet that contains ultrafine fibers with average single fiber diameters of 0.3 to 7 and having naps on the surface, with the goal of providing a leather-like sheet with a satisfactory surface touch and excellent coloring properties and appearance quality for extended periods, and a method for producing it, wherein in terms of naps present within 0.2 μm from the sheet surface in the thickness direction, for the minimum interfiber distances between each fiber among 100 randomly extracted fibers and its nearest adjacent nap, the average value for the 100 fibers is 10 to 30 and when 20 surrounding naps are selected in order of closer distance from each fiber among 100 randomly extracted fibers, the standard deviation for their interfiber distances for the 100 fibers is 10 or lower. The sheet is obtained by a process comprising the following step 1: removal of the sea-component from the fiber sheet containing sea-island fibers to obtain ultrafine fibers; step 2: alkali reduction treatment of the ultrafine fibers of the fiber sheet containing ultrafine fibers; step 3: addition of a elastomer to the obtained fiber sheet after steps 1 and 2; and step 4: slicing the sheet in half horizontally after step 3, and buffing treatment of at least one side of the sliced sheet. PTL 5 states that it is preferred to uniformly disperse the ultrafine fibers after alkali reduction treatment of the ultrafine fibers, and that the treatment for uniform dispersion of the ultrafine fibers is a method of showering the fiber sheet with water after alkali reduction treatment, a method of immersing the fiber sheet in water and dispersing the fibers while contacting them with a water stream in a Vibrowasher or the like, or a method of dispersing the fibers while contacting them with a water stream using a water jet punch, that the treatment for uniform dispersion of the ultrafine fibers causes the ultrafine fibers to be homogeneously distributed, and that by using fibers with average single fiber diameters of 0.3 to 7 μm and satisfactory coloring properties it is possible to obtain a sheet that has the naps in a well-dispersed state for long periods without temporary surface-touch modification by chemical agents, and exhibits a satisfactory surface touch property (a non-rough feel), uniform naps, delicate appearance quality and excellent coloring properties.

However, in dispersion treatment while contacting with a water stream using a Vibrowasher or the like for uniform dispersion of ultrafine fibers as described in PTL 5, the resulting fiber dispersion is not sufficient and the PU resin dispersibility is also inferior, with an unsatisfactory luxuriant feel and slick feel as well, while it is unclear what manner of texture (stiffness) is exhibited.

In the current state of the art, therefore, various attempts have been made to provide artificial leather obtained by impregnating a PU resin into a fiber sheet that comprises sea-island fibers, wherein the fiber dispersibility and PU resin dispersibility are increased to provide an artificial leather with excellent texture (stiffness), luxuriant feel (fiber bundle dispersibility) and slick feel (suitable sizes of the PU resin masses), but a trade-off exists between the texture (stiffness), luxuriant feel and slick feel (suitable sizes of the PU resin masses), and consequently it still remains an unachieved goal to obtain artificial leather of a quality level that can satisfy all of the properties of texture (stiffness), luxuriant feel (fiber bundle dispersibility) and slick feel (suitable sizes of the PU resin masses), and a method for producing it.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Patent Publication No. 4089324 -   [PTL 2] Japanese Unexamined Patent Publication No. 2014-25165 -   [PTL 3] International Patent Publication No. WO2015/129602 -   [PTL 4] Japanese Unexamined Patent Publication No. 2017-8478 -   [PTL 5] Japanese Unexamined Patent Publication No. 2015-161040

SUMMARY Technical Problem

In light of the problems of the prior art described above, the problem to be solved by the invention is that of providing artificial leather with minimal harm to the human body and environment, and having a level of quality that can satisfactory all of the properties of texture (stiffness), luxuriant feel (fiber bundle dispersibility) and slick feel (suitable sizes of the PU resin masses), as well as a method for producing it.

Solution to Problem

As a result of diligent experimentation with the aim of solving this problem, the present inventors have found, unexpectedly, that the problem can be solved by the artificial leather having the following features.

Specifically, the present invention provides the following.

[1] Artificial leather comprising a fiber sheet and a polyurethane resin, wherein the fiber sheet contains at least a fiber layer (A) constituting the first outer surface of the artificial leather, the k-nearest neighbor distance (k=9, radius r=20 μm) between cross-sections of the single fibers forming the fiber layer (A) in a cross-section of the artificial leather in the thickness direction is 10% to 80%, the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 10% to 30%, and the standard deviation for the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 25% or lower.

[2] The artificial leather according to [1] above, wherein in a three-dimensional image obtained by X-ray CT of the fiber layer (A), the average value of space sizes, as the diameters of maximum spheres that fit in the spaces excluding the fibers forming the fiber layer (A) and the polyurethane resin (average space size), is 5 μm to 35 μm in the thickness direction of the fiber layer (A).

[3] The artificial leather according to [1] or [2] above, wherein the fiber sheet has a construction of two or more layers comprising a fiber layer (A) constituting the first outer surface, and a scrim and/or fiber layer (B) contacting the fiber layer (A).

[4] The artificial leather according to any one of [1] to [3] above, wherein the mean diameter of the single fibers composing the fiber layer (A) is 1.0 μm to 8.0 μm.

[5] The artificial leather according to any one of [1] to [4] above, wherein the polyurethane resin is a water-dispersed polyurethane resin.

[6] The artificial leather according to any one of [1] to [5] above, wherein the adhesion rate of the polyurethane resin on the fiber sheet is 15 wt % to 50 wt %.

[7] The artificial leather according to any one of [1] to [6] above, wherein the stiffness is 28 cm or lower.

[8] The artificial leather according to any one of [1] to [7] above, wherein the fiber sheet is composed of polyester fibers.

[9] The artificial leather according to any one of [1] to [8] above, wherein the luxuriant feel is grade 4.0 or higher.

[10] A method for producing the artificial leather according to any one of [1] to [9] above, which comprises the following steps:

a step of forming a fiber web of sea-island cut fibers, needle punching it, and then dissolving of the sea-component of the fiber sheet, to obtain a fiber sheet with the island-component single fibers exposed; and

a step of subjecting the obtained fiber sheet to water flow dispersion treatment to obtain a fiber sheet with the single fibers dispersed.

[11] The method according to [10] above, which further comprises the following steps:

a step of impregnating a water-dispersed polyurethane resin dispersion containing hot water-soluble resin microparticles into the fiber sheet with the single fibers dispersed, and then coagulating the polyurethane resin by heating to obtain a sheet filled with the polyurethane resin, and:

a step of using hot water to remove the hot water-soluble resin microparticles from the obtained sheet.

[12] The method according to [10] or [11] above, wherein the hot water-soluble resin microparticles are composed of a polyvinyl alcohol resin.

[13] The method according to any one of [10] to [12] above, wherein the water flow dispersion treatment is carried out using a plurality of nozzles having nozzle hole intervals of 1.0 mm or smaller and nozzle hole diameters of 0.05 mm to 0.30 mm.

[14] The method according to any one of [10] to [13] above, wherein the water flow dispersion treatment is carried out using a plurality of nozzles that discharge a water stream with a disturbance of 10% or greater.

[15] The method according to any one of [11] to [14] above, wherein the solid concentration of the water-dispersed polyurethane resin dispersion is 10 wt % to 35 wt %.

[16] The method according to any one of [11] to [15] above, wherein the content of hot water-soluble resin microparticles in the water-dispersed polyurethane resin dispersion is 1 wt % to 20 wt %.

Advantageous Effects of Invention

The artificial leather of the invention has excellent texture (stiffness), luxuriant feel (fiber bundle dispersibility) and slick feel (suitable sizes of the polyurethane resin masses), and can therefore be satisfactorily used as a sheet covering material or interior finishing material for interiors, automobiles, aircraft or railway vehicles, or in a clothing product.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual drawing showing an example of the construction of artificial leather indicated as reference numeral 1. Since the scrim indicated by reference numeral 11 and the fiber layer (B) indicated by reference numeral 13 are optional, the artificial leather of this embodiment may be a single layer of the fiber layer (A) indicated by reference numeral 12, or two layers consisting of the fiber layer (A) and the scrim or fiber layer (B), or three layers consisting of the fiber layer (A), the scrim and the fiber layer (B).

FIG. 2 is a conceptual drawing illustrating how the mean diameter of the fibers composing the fiber layer (A) is determined.

FIG. 3 is a conceptual drawing illustrating the single fiber cross-sectional k-nearest neighbor distance (%) for cross-sections in the thickness direction from fiber layer (A), the cross-sectional PU resin area ratio, the single fiber mean diameter, the surface PU resin area ratio on the first outer surface, and the sampling location for each space size.

FIG. 4 is a pair of images showing manual marking of each single fiber cross-section in a prescribed image region, in order to determine the single fiber cross-sectional k-nearest neighbor distance (%) for a cross-section in the thickness direction.

FIG. 5 is a conceptual drawing illustrating how to determine the single fiber cross-sectional k-nearest neighbor distance (%) for a cross-section in the thickness direction.

FIG. 6 is a conceptual drawing illustrating how to determine the cross-sectional or surface PU resin area ratio, and its standard deviation.

FIG. 7 is a graph showing cross-sectional PU resin area ratios (%) and standard deviation for the Examples and Comparative Examples.

FIG. 8 is a conceptual drawing showing nozzle hole intervals for one and two nozzle hole rows of nozzles used for water flow dispersion treatment.

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will now be explained in detail with the understanding that the invention is not limited to the embodiments. Unless otherwise specified, the values mentioned throughout the present disclosure are values obtained by the methods described herein under “Examples” or methods known to be equivalent to them by those skilled in the art.

<Artificial Leather>

A first embodiment of the invention is artificial leather comprising a fiber sheet and a PU resin, wherein the fiber sheet contains at least a fiber layer (A) constituting the first outer surface of the artificial leather, the k-nearest neighbor distance (k=9, radius r=20 μm) between cross-sections of the single fibers forming the fiber layer (A) in a cross-section of the artificial leather in the thickness direction is 10% to 80%, the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 10% to 30%, and the standard deviation for the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 25% or lower.

Throughout the present specification, “artificial leather” is that defined according to the Household Goods Quality Labeling Law as “leather comprising a special nonwoven fabric (mainly a fiber layer having a random three-dimensional spatial structure, impregnated with a PU resin or a elastomer of similar flexibility) as the base material”. In the definition according to JIS-6601, artificial leather is classified as “smooth” having the grain side appearance of leather, and “napped” having a leather suede or velour outer appearance, but the artificial leather of this embodiment is classified as “napped” (that is, suede-like artificial leather having a brushed-style outer appearance). A suede-like outer appearance can be formed by buffing treatment (raising treatment) of the outer surface of the fiber layer (A) (the side that is to be the first outer surface of the artificial leather) using sandpaper or the like. For the purpose of the present specification, the first outer surface of the artificial leather is the surface that is externally exposed when the artificial leather is used (the surface that contacts with the human body, in the case of a chair, for example) (see FIG. 1 and FIG. 3). According to one aspect, for suede-like artificial leather the first outer surface has raised or naps produced by buffing, for example.

Unless otherwise specified, the term “fiber web” used herein refers to the state before entangling of cut fibers, the term “fiber sheet” refers to the state after entangling and before PU resin filling, the term “sheet” refers to the state after PU resin filling and before dye finishing, and the term “artificial leather” refers to the state of the product after dye finishing. The term “nonwoven fabric” encompasses “fiber web”, “fiber sheet”, “sheet” and “artificial leather”, and the term “fibrous base material” encompasses woven or knitted fabrics in addition to “nonwoven fabric”.

[k-Nearest Neighbor Distance (k=9, Radius r=20 μm) Between Single Fiber Cross-Sections Composing Fiber Layer (A)]

One feature of this embodiment is that the k-nearest neighbor distance (k=9, radius r=20 μm) between cross-sections of the single fibers forming the fiber layer (A) in a cross-section of the artificial leather in the thickness direction is 10% to 80%. The k-nearest neighbor distance (k=9, radius r=20 μm) is an index of the compactness of single fibers.

The measuring method used will be described below, but the k-nearest neighbor algorithm is a method where the decision boundary is the kth nearest radius in Euclidean distance (the square root of the sum of squares of the distances in the X-direction and Y-direction (=shortest distance)) for k single fiber cross-sections near one arbitrary single fiber cross-section, and for this embodiment an SEM images are taken, and it is determined whether or not the k=9th nearest single fiber cross-section is present within the distance of a 20 μm radius from the approximate center of one arbitrary single fiber cross-section. This is determined for all of the single fiber cross-sections in a single SEM image, and the single fiber cross-section k=9 nearest neighbor distance ratio (%) is calculated by the following formula:

Single fiber cross-section (k=9) nearest neighbor distance ratio (%)={(Number of single fiber cross-sections where k=9th nearest single fiber cross-section is present within distance of 20 μm radius from approximate center of single fiber cross-section)/(total number of single fiber cross-sections within one SEM image)}×100.

If the k-nearest neighbor distance ratio (k=9, radius r=20 μm) between single fiber cross-sections composing the fiber layer (A) in a cross-section in the thickness direction of the artificial leather is 10% or higher, then the single fibers are in a suitably dispersed state and consequently the PU resin masses of the fiber layer (A) are also suitably dispersed, so that the suitably dispersed PU resin masses are felt when the artificial leather is touched with the fingertip and a satisfactory slick feel (PU resin mass sizes) is perceived. If the k-nearest neighbor distance ratio (k=9, radius r=20 μm) is 80% or lower, on the other hand, the single fibers are suitably aggregated, thus resulting in a smooth tactile sensation with a high luxuriant feel (high fiber bundle dispersibility). The k-nearest neighbor distance ratio (k=9, radius r=20 μm) is preferably 20% to 70% and more preferably 30% to 60%.

The k-nearest neighbor distance ratio (k=9, radius r=20 μm) can be controlled to 80% or lower by a step of forming a fiber web of sea-island cut fibers, and subsequent needle punching treatment and dissolving of the sea-component of the obtained fiber sheet to obtain a fiber sheet with the island-component single fibers exposed, followed by a step of subjecting the obtained fiber sheet to water flow (or water punching) dispersion treatment, to obtain a fiber sheet with the single fibers dispersed, as described below. The water flow dispersion treatment is preferably carried out by spraying of high-pressure water using a plurality of nozzles having nozzle hole intervals of 1.00 mm or smaller. As shown in FIG. 8, the nozzle hole interval is the distance in the nozzle width direction between a nozzle hole and the nozzle hole most adjacent to that nozzle hole in the nozzle width direction (whether nozzle holes are in a single row or in two or more rows). If the nozzle hole interval is 1.00 mm or smaller it will be possible to discharge a water stream with compact intervals onto the fiber sheet, dispersing the single fibers as single fiber bundles and helping to improve the luxuriant feel and slick feel. Water stream trajectories generated by the water flow dispersion treatment will also be less visible on the fiber sheet surface. The nozzle hole interval is preferably 0.60 mm or smaller and even more preferably 0.30 mm or smaller.

The number of rows of nozzle holes opened in the widthwise direction of the water flow dispersion apparatus may be one or more rows. For water flow dispersion treatment, it is common to remove the water that has been loaded into the fiber sheet during water flow dispersion treatment, from the viewpoint of uniformity and shape stability retention of the fiber sheet, and the dewatering is carried out by a suction method from the side opposite from the water flow dispersion treatment side. In this case, if the nozzle hole intervals are narrowed with a single nozzle hole row, for example, the dewatering performance is not sufficient for the amount of loaded water, often resulting in impaired uniformity and shape stability of the fiber sheet. Therefore, multiple rows are provided, with the nozzle hole interval per single nozzle hole row widened to reduce the amount of water loaded per nozzle hole row, as a preferred means for helping to balance the amount of loaded water with the dewatering performance. For example, when a dewatering fault has occurred with a single nozzle row having a nozzle hole interval of 0.30 mm, and there are two nozzle rows with nozzle hole intervals of 0.60 mm in each row, then if the second row is placed so that nozzle hole rows with nozzle intervals of 0.60 mm are placed with a phase difference of 0.30 mm with respect to the first row, the water flow trajectory (nozzle hole interval) will be 0.30 mm, and an effect of improving dewatering faults can be obtained. The nozzle hole interval is also preferably widened and multiple rows provided in order to facilitate nozzle processing. The nozzle hole intervals (water flow trajectory) are preferably equal spacings in order to help produce uniform dispersion of the single fibers and prevent the water flow trajectories from being visible to allow improvement in the luxuriant feel and slick feel.

The inter-row distance of the nozzle holes in the case of a plurality of nozzle hole rows is preferably a distance corresponding to the nozzle hole intervals within the first nozzle row, for example, from the viewpoint of dewaterability.

The hole diameters of high-pressure water injection nozzle holes in water flow dispersion treatment are preferably 0.05 mm to 0.30 mm, more preferably 0.05 mm to 0.20 mm and even more preferably 0.08 mm to 0.13 mm, from the viewpoint of facilitating high dispersion of the single fibers, reducing visibility of water flow trajectories, and helping to obtain balance with the dewatering performance without an excessive amount of loaded water.

The water pressure of the water flow dispersion treatment is preferably spraying at 1.0 to 10.0 MPa. If the water pressure in the water flow dispersion treatment is 1.0 MPa or greater, the single fibers in the form of single fiber bundles will be more easily dispersed, and by limiting the water pressure in water flow dispersion treatment to no higher than 10.0 MPa, over-dispersion of the cut fiber bundles can be avoided to allow easier control of the k-nearest neighbor distance ratio to 10% to 80%. The single fibers in single fiber bundle form can also be dispersed, helping to reduce visibility of water flow trajectories. A high water pressure during water flow dispersion treatment can sometimes cause the water stream to perforate the fiber sheet so that the energy is not used to disperse the single fiber bundles, and may instead result in a reduced dispersing effect for the single fiber bundles compared to low water pressure. With a high water pressure in water flow dispersion treatment the fiber sheet increases in density and tends to have a poorer texture (stiffness). The water pressure for dispersion treatment is more preferably 1.5 to 7.0 MPa and even more preferably 2.0 to 4.5 MPa.

The manner of water stream discharge from the nozzle holes is preferably by using a plurality of nozzles to discharge a water stream with disturbance of 10% or greater. Disturbance is an index of fluctuation in the diameter of a water stream. In order to efficiently convert the water stream energy to fiber dispersion, the disturbance is preferably 12% or greater and even more preferably 15% or greater. The disturbance is calculated by the following formula:

Disturbance (%)=σ (mm)/W (mm)×100

where W is the mean diameter of the water stream in a range of 25 mm to 35 mm from the discharge point of the nozzle hole, and σ is the standard deviation of the mean diameter.

While the mechanism of dispersion of the single fiber bundles by disturbance is not fully understood, the present inventors believe that a larger disturbance, compared to a smaller one, allows the water stream energy to be applied in the direction perpendicular to the fiber sheet and allows dispersion to occur more easily in all directions in the horizontal direction, thereby efficiently converting the water stream energy into single fiber bundle dispersing energy for a greater dispersion effect. For example, it is thought that with high water pressure the water stream energy that passes through the fiber sheet and is lost is more easily taken in as dispersion energy.

Moving the high-pressure water spraying nozzle with circular motion, or with reciprocal movement perpendicular to the processing direction (machine direction), is also preferred in terms of promoting dispersion of the single fibers and improving the luxuriant feel or slick feel.

The distance from the high-pressure water spraying surface to the product being treated is preferably 5 mm to 100 mm, more preferably 10 mm to 60 mm and even more preferably 20 mm to 40 mm, from the viewpoint of the single fiber bundle dispersing effect and of fabric guiding before water flow dispersion treatment, and processing throughput during the water flow dispersion treatment.

[Cross-Sectional PU Resin Area Ratio in Cross-Section of the Fiber Layer (A) in the Thickness Direction, and Standard Deviation]

In the artificial leather of this embodiment, the cross-sectional PU resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 10% to 30%, and the standard deviation of the cross-sectional PU resin area ratio is 25% or lower.

If the cross-sectional PU resin area ratio exceeds 30% the adhesion rate of PU resin will be too high, resulting in a stronger rubber-like feel for the artificial leather. This reduces the flexibility and produces a hard texture. When the artificial leather has no scrim, the cross-sectional PU resin area ratio is 10% or greater from the viewpoint of more easily obtaining adequate mechanical properties in the horizontal direction. The cross-sectional PU resin area ratio is preferably 15% to 30%, more preferably 15% to 28% and even more preferably 15% to 26%.

The cross-sectional PU resin area ratio indicates the texture (stiffness) as described below, in accordance with the k-nearest neighbor distance ratio (k=9, radius r=20 μm). For example, excessive single fiber bundles are present when the k-nearest neighbor distance ratio (k=9, radius r=20 μm) exceeds 80%. However, the water-dispersed PU resin will have a greater tendency to be coagulated while adhering to the single fibers or single fiber bundles. In other words, when the cross-sectional PU resin area ratio is 10% or greater in the presence of excess single fiber bundles with a k-nearest neighbor distance ratio of 80% or greater, the PU resin masses aggregate and adhere to the single fiber bundles, impairing the texture (stiffness). If the k-nearest neighbor distance ratio (k=9, radius r=20 μm) is 10% to 80% and the cross-sectional PU resin area ratio is 10% to 30%, the texture (stiffness) is 28 cm or lower.

As explained below (see FIG. 6), the cross-sectional PU resin area ratio is determined by binarizing the PU resin in an SEM image as black portions, calculating the area ratio of PU resin for partitions from the binarized image by the partition method, and averaging the cross-sectional PU resin area ratio (%) for all of the partitions, while the standard deviation indicates variation from the average for each partition. If the standard deviation of the cross-sectional PU resin area ratio is 25% or lower, the distribution of PU resin mass sizes will be controlled, thus reducing variation in the texture (stiffness). The standard deviation for the cross-sectional PU resin area ratio is preferably 25% or lower, more preferably 22% or lower and even more preferably 20% or lower. While the lower limit for the standard deviation of the cross-sectional PU resin area ratio is not particularly restricted it may be 0% or higher.

As explained below, the cross-sectional PU resin area ratio can be controlled to 10% to 30%, for example, by impregnating a water-dispersed PU resin dispersion containing hot water-soluble resin microparticles (such as PVA resin fine particles), and then coagulating the PU resin by heating to obtain a sheet filled with the PU resin. Alternatively, by carrying out a step of forming a fiber web with sea-island cut fibers and then removing the sea-component from the fiber sheet after needle punching treatment to obtain a fiber sheet with the island-component single fibers exposed, followed by a step of water flow dispersion treatment of the fiber sheet to obtain a fiber sheet with the single fibers dispersed, it is possible to simultaneously disperse the PU resin adhering to the fibers as the single fibers are dispersed, thereby controlling the standard deviation of the cross-sectional PU resin area ratio to 25% or lower.

[Average Space Size]

In the artificial leather of this embodiment, in a three-dimensional image obtained by X-ray CT of the fiber layer (A), the average value of space sizes, as the diameters of maximum spheres that fit in the spaces excluding the fibers composing the fiber layer (A) and the PU resin (average space size), is preferably 5 μm to 35 μm in the thickness direction of the artificial leather.

As explained below, the average space size is the average value in the thickness direction for the diameters (μm) of maximum spheres that fit in the spaces excluding the single fibers composing the fiber layer (A) and the PU resin, as seen in a three-dimensional image of the fiber layer (A) by X-ray CT. The average space size is an indicator of the dispersed state of structures composed of fibers and PU resin masses in the fiber layer (A) of the artificial leather. A larger average space size means that the fibers and PU resin masses are closely adhering together. If the average space size is in the range of 5 μm to 35 μm, the fibers and PU resin will be suitably dispersed, to more easily provide texture (stiffness) of 28 cm or lower. As explained below, the average space size of the artificial leather as the final product can be controlled to 5 μm to 35 μm, for example, by impregnating the fiber sheet with the single fibers dispersed with a water-dispersed PU resin dispersion containing hot water-soluble resin microparticles (such as PVA resin fine particles), and then coagulating the PU resin by heating to obtain a fiber sheet filled with the PU resin. The average space size is more preferably 5 μm to 25 μm and even more preferably 5 μm to 13 μm.

[Adhesion Rate of PU Resin in Fiber Sheet]

In the artificial leather of this embodiment, the adhesion rate of the PU resin in the fiber sheet is preferably 15 wt % to 50 wt %, more preferably 22 wt % to 45 wt % and even more preferably 26 wt % to 40 wt %. The proportion of PU resin with respect to the fiber sheet affects how the cross-sectional PU resin area ratio and the average space size are controlled. With a low PU resin ratio, the cross-sectional PU resin area ratio tends to be low and the average space size tends to be large. With a high PU resin ratio, on the other hand, the cross-sectional PU resin area ratio tends to be high and the average space size tends to be small. A PU resin ratio with respect to the fiber sheet of 15 wt % or greater will allow the fibers to be satisfactorily held by the PU resin, so that mechanical strength including abrasion resistance on a level satisfying commercial demand can be more easily obtained. A PU resin ratio of 50 wt % or lower with respect to the fiber sheet will tend to result in a soft hand quality.

[Polyurethane (PU) Resin]

The PU resin is preferably obtained by reacting a polymer diol with an organic diisocyanate and a chain extender.

Examples of polymer diols to be used include polycarbonate-based, polyester-based, polyether-based, silicone-based and fluorine-based diols, and copolymers of any two or more of these may be used. From the viewpoint of hydrolysis resistance, it is preferred to use a polycarbonate-based or polyether-based diol, or a combination thereof. From the viewpoint of light fastness and heat resistance, polycarbonate-based or polyester-based diols, or their combinations, are preferred. From the viewpoint of cost competitiveness, a polyether-based or polyester-based diol, or a combination thereof, is preferred.

A polycarbonate-based diol can be produced by transesterification reaction of an alkylene glycol and a carbonic acid ester, or by reaction between phosgene or a chlorformic acid ester and an alkylene glycol.

Examples of alkylene glycols include straight-chain alkylene glycols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol and 1,10-decanediol; branched alkylene glycols such as neopentyl glycol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol and 2-methyl-1,8-octanediol; alicyclic diols such as 1,4-cyclohexanediol; and aromatic diols such as bisphenol A; used either alone, or in any combinations of two or more.

Polyester-based diols include polyester diols obtained by condensation between any of various low molecular weight polyols and polybasic acids.

Examples of low molecular weight polyols include one or more selected from among ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butanediol, 1,4-butanediol, 2,2-dimethyl-1,3-propanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 1,8-octanediol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, cyclohexane-1,4-diol and cyclohexane-1,4-dimethanol. An addition product of an alkylene oxide with bisphenol A may also be used.

Examples of polybasic acids include one or more selected from the group consisting of succinic acid, maleic acid, adipic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, dodecanedicarboxylic acid, phthalic acid, isophthalic acid, terephthalic acid and hexahydroisophthalic acid.

Examples of polyether-based diols include polyethylene glycol, polypropylene glycol, polytetramethylene glycol, and copolymer diols comprising their combinations.

The number-average molecular weight of the polymer diol is preferably 500 to 4000. The number-average molecular weight is 500 or higher and more preferably 1500 or higher to help prevent hardening of the texture. The number-average molecular weight is 4000 or lower and more preferably 3000 or lower to help maintain satisfactory strength of the PU resin.

Examples of organic diisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, isophorone diisocyanate and xylylene diisocyanate; and aromatic diisocyanates such as diphenylmethane diisocyanate and tolylene diisocyanate, and their combinations may also be used. Preferred among these from the viewpoint of light fastness are aliphatic diisocyanates such as hexamethylene diisocyanate, dicyclohexylmethane diisocyanate and isophorone diisocyanate.

Chain extenders to be used include amine-based chain extenders such as ethylenediamine and methylenebisaniline, or diol-based chain extenders such as ethylene glycol. A polyamine obtained by reacting a polyisocyanate and water may also be used as a chain extender.

The PU resin may also be used in the form of a solvent-type PU resin with the PU resin dissolved in an organic solvent such as N,N-dimethylformamide, or a water-dispersed PU resin with the PU resin emulsified with an emulsifying agent and dispersed in water. A water-dispersed PU resin is preferred among these from the viewpoint of obtaining a finer form of the PU resin filling the fiber sheet, more easily obtaining the performance required for artificial leather such as texture and mechanical properties even with smaller amounts of addition, and not requiring the use of organic solvents so that the environmental load can be reduced. Since the water-dispersed PU resin can be impregnated into a fiber sheet in the form of a dispersion with the PU resin dispersed with the desired particle size (hereunder also referred to as “PU resin dispersion”), the particle size can be controlled to satisfactorily manage the manner in which the PU resin fills the fiber sheet.

The water-dispersed PU resin used may be a self-emulsifying PU resin containing hydrophilic groups within the PU molecules, or a forced-emulsifying PU resin in which the PU resin has been emulsified with an external emulsifying agent.

A crosslinking agent may also be used with the water-dispersed PU resin in order to improve the durability, including resistance to moist heat, abrasion resistance and hydrolysis resistance. A crosslinking agent is also preferably added to improve the durability during jet dyeing, reduce loss of the fibers and obtain excellent surface quality. The crosslinking agent may be an external crosslinking agent added as an addition component to the PU resin, or it may be a reactive group-introducing internal crosslinking agent that can produce a crosslinked structure beforehand within the PU resin structure.

Since the water-dispersed PU resin used in the artificial leather will generally have a crosslinked structure to provide dyeing resistance, it tends to be poorly soluble in organic solvents such as N,N-dimethylformamide. Therefore when observing a cross-section with an electron microscope, for example, if a resinous substance remains which lacks the form of the fibers after immersion of the artificial leather in N,N-dimethylformamide at room temperature for 12 hours to dissolve the PU resin, the resinous substance may be judged to be the water-dispersed PU resin.

According to a preferred embodiment, from the viewpoint of easily controlling the cross-sectional PU resin area ratio and its standard deviation, as well as the average space size, the PU resin is filled using a PU resin dispersion, and the mean primary particle size of the PU resin in the dispersion is preferably 0.1 μm to 0.8 μm, more preferably 0.1 μm to 0.6 μm and even more preferably 0.2 μm to 0.5 μm. The mean primary particle size is the value obtained by measurement of the PU resin dispersion using a laser diffraction particle size distribution analyzer (“LA-920” by Horiba, Ltd.). If the mean primary particle size of the PU resin is 0.1 μm or greater, then the force with which the PU resin holds the fibers together in the fiber sheet (binding force) will be satisfactory, allowing artificial leather with excellent mechanical strength to be obtained. Limiting the mean primary particle size of the PU resin to 0.8 μm or smaller will inhibit aggregation and coarseness of the PU resin, which is advantageous for controlling the standard deviation of the cross-sectional PU resin area ratio to 25% or lower. If the mean primary particle size of the PU resin in the PU resin dispersion is 0.1 μm to 0.8 μm, then the fibers composing the artificial leather (surface layer) will be held together at more points, allowing a soft hand quality (stiffness) and excellent mechanical strength (such as abrasion resistance) to be obtained.

[Solid Concentration of PU Resin Dispersion]

According to a typical aspect, as explained below, the PU resin is impregnated as an impregnating liquid in the form of a solution (dissolved in a solvent) or dispersion (water-dispersed). The solid concentration of the water-dispersed PU resin dispersion may be 10 wt % to 35 wt %, for example, and is preferably 15 to 30 wt % and more preferably 15 to 25 wt %. According to one aspect, the impregnating liquid is prepared and impregnated into the fiber sheet so that the adhesion rate of the PU resin with respect to 100 wt % of the fiber sheet is 15 wt % to 50 wt %.

The impregnating liquid containing the PU resin (such as the water-dispersed PU resin) may also contain additives such as stabilizers (ultraviolet absorbers and antioxidants), flame retardants, antistatic agents and pigments (such as carbon black), as necessary. The total amount of additives in the artificial leather may be 0.1 to 10.0 parts by mass, 0.2 to 8.0 parts by mass or 0.3 to 6.0 parts by mass, for example, with respect to 100 parts by mass of the PU resin. Such additives become distributed in the PU resin of the artificial leather. The values of the size of the PU resin and the mass ratio with respect to the fiber sheet mentioned herein are assumed to include the additives (when used).

[Hot Water-Soluble Resin Microparticles]

When an impregnating liquid containing the PU resin is to be impregnated into a fiber sheet to fill the fiber sheet with the PU resin, preferably the water-dispersed PU resin dispersion containing the hot water-soluble resin microparticles is impregnated into the fiber sheet and the PU resin is subsequently coagulated by heating to obtain the sheet filled with the PU resin. In the finishing-process steps or dyeing step, the hot water-soluble resin microparticles are removed from the fiber sheet using hot water, thus dividing part of the continuous layer of the PU resin and forming pores, to micronize the state of adhesion of the PU resin.

The hot water-soluble resin microparticles may be partially saponified PVA resin fine particles or completely saponified PVA resin fine particles. Since completely saponified PVA resin fine particles tend to be less elutable into water at ordinary temperature (or room temperature) (20° C.) compared to partially saponified PVA resin fine particles, completely saponified PVA resin fine particles are preferably used as the hot water-soluble resin microparticles. From the viewpoint of inhibiting elution into water at ordinary temperature (20° C.), the saponification degree of completely saponified PVA resin fine particles is preferably 95 mol % or greater and more preferably 98 mol % or greater. In order to achieve both force for binding to and holding the fibers and micronization of the adhered state of the PU resin, the mean particle size (size) of the hot water-soluble resin microparticles is preferably 1 μm to 8 μm, more preferably 2 μm to 6 μm and even more preferably 2 μm to 4 μm. If the mean particle size is 1 μm or greater the hot water-soluble resin microparticles will be less likely to aggregate, and if the mean particle size is 8 μm or smaller the hotwater-soluble resin microparticles will be more satisfactorily impregnated into the fiber sheet. The microparticles used may be “NL-05” by Mitsubishi Chemical Holdings Corp., and micronization of the hot water-soluble resin microparticles may be by the method described in Japanese Unexamined Patent Publication HEI No. 7-82384.

The content of hot water-soluble resin microparticles in the water-dispersed PU resin dispersion is preferably 1 wt % to 20 wt %, more preferably 2 wt % to 15 wt % and even more preferably 3 wt % to 10 wt %. A content of 1 wt % or greater of the hot water-soluble resin microparticles in the water-dispersed PU resin dispersion will tend to promote dispersion of the PU resin masses. A content of 20 wt % or lower of the hot water-soluble resin microparticles in the water-dispersed PU resin dispersion, on the other hand, will tend to maintain a stable dispersion without aggregation of the microparticles.

As used herein, “hot water-soluble resin” refers to a resin that is poorly soluble in water at ordinary temperature.

[Hot Water-Soluble Resin]

When the water-dispersed PU resin dispersion containing the hot water-soluble resin microparticles is to be impregnated into the fiber sheet and the PU resin subsequently coagulated by heating to obtain the sheet filled with the PU resin, a step of adhering the hot water-soluble resin to the fiber sheet may be carried out before the water-dispersed PU resin dispersion containing the hot water-soluble resin microparticles is impregnated into the fiber sheet. The method of adhering the hot water-soluble resin (such as a PVA resin) may be preparation of an aqueous solution of the hot water-soluble resin followed by impregnation of the fiber sheet with the water-soluble solution and drying. Removing the hot water-soluble resin together with the hot water-soluble resin microparticles from the fiber sheet using hot water in the finishing-process steps or dyeing step can inhibit adhesion between the fibers and PU resin or can divide part of the continuous layer of the PU resin and form pores to micronize the state of adhesion of the PU resin, thus tending to improve the texture of the artificial leather.

The hot water-soluble resin may be a partially saponified PVA resin or completely saponified PVA resin. Since a completely saponified PVA resin tends to be less elutable into water at ordinary temperature (20° C.) compared to a partially saponified PVA resin, a completely saponified PVA resin is preferably used as the hot water-soluble resin. From the viewpoint of inhibiting elution into water at ordinary temperature (20° C.), the saponification degree of the completely saponified PVA resin is preferably 95 mol % or greater and more preferably 98 mol % or greater. In order to increase the permeability of the aqueous hot water-soluble resin solution during impregnation, the polymerization degree is preferably 1000 or lower and more preferably 700 or lower.

[Fiber Sheet]

As shown in FIG. 1, the fiber sheet 1 includes at least the fiber layer (A) 12, while the scrim 11 and fiber layer (B) 13 are optional and are not essential elements. The artificial leather of this embodiment may therefore be a single layer of the fiber layer (A), or two layers consisting of the fiber layer (A) and the scrim or fiber layer (B), or three layers consisting of the fiber layer (A), the scrim and the fiber layer (B).

When a scrim 11 and/or fiber layer (B) 13 are not included, the fiber layer (A) may be a single-layer fiber sheet which is sliced in half horizontally and then filled with the PU resin, as explained below. According to one aspect, the fiber sheet has a monolayer structure with no scrim. This will allow productivity to be increased by slicing in half horizontally.

According to another aspect, the fiber sheet has a three-layer structure with a scrim as the intermediate layer. For example, a woven or knitted fabric scrim 11 may be inserted in a sandwich manner between the fiber layer (A) 12 constituting the first outer surface of the artificial leather and the fiber layer (B) 13 constituting the second outer surface of the artificial leather, forming a three-layer structure with the fibers tangled between the layers, which is preferred for dimensional stability, tensile strength and tear strength. A three-layer structure of the fiber layer (A), fiber layer (B) and a scrim inserted between them also allows the fiber layer (A) and fiber layer (B) to be have separate designs, and is therefore preferred from the viewpoint of free customization of the diameters and types of fibers composing each layer to match the functions and usages required for artificial leather. For example, using ultrafine fibers for fiber layer (A) and flame-retardant fibers for fiber layer (B) allows both excellent surface quality and high flame retardance to be obtained.

When the fiber sheet includes a scrim, a woven or knitted fabric scrim is preferably made of a polymer of the same type as the fibers composing the fiber layer (A), from the viewpoint of obtaining the same color by dyeing. If the fibers of the fiber layer (A) are polyester-based, for example, the fibers of the scrim are also preferably polyester-based, and when the fibers of the fiber layer (A) are polyamide-based the fibers of the scrim are also preferably polyamide-based. For a knitted fabric scrim, it is preferably a single knit of 22 gauge to 28 gauge. A woven fabric scrim can exhibit higher dimensional stability and strength than a knitted fabric. The woven fabric texture may be a plain weave, twill weave or satin weave, but is preferably a plain weave from the viewpoint of cost and entangling properties.

The yarn composing the woven fabric may be monofiber or multifiber yarn. The single fiber fineness of the yarn is preferably 5.5 dtex or smaller to more easily obtain flexible artificial leather. The form of the yarn composing the woven fabric may be multifiber gray yarn of polyester or polyamide, or false twisted finished yarn with added twisting of 0 to 3000 T/m. When multifibers are used they may be common ones, and are preferably 33 dtex/6f, 55 dtex/24f, 83 dtex/36f, 83 dtex/72f, 110 dtex/36f, 110 dtex/48f, 167 dtex/36f or 166 dtex/48f polyester or polyamide, for example. The yarn composing a woven fabric may be multifiber long fibers. The woven density of the yarn in a woven fabric is preferably 30 to 150/inch and more preferably 40 to 100/inch from the viewpoint of obtaining artificial leather that is flexible with excellent mechanical strength. In order to obtain satisfactory mechanical strength and suitable texture, the basis weight of the woven fabric is preferably 20 to 150 g/m². The presence or absence of false twisting, the number of twists, the multifiber single fiber fineness and woven density for a woven fabric also contribute to the entangling properties between the fibers composing the fiber layer (A) and the optional fiber layer (B), and to the mechanical properties including flexibility, seam strength, tearing strength, tensile strength and elongation and stretchability of the artificial leather, and they may be selected as appropriate for the desired physical properties and intended use.

From the viewpoint of obtaining artificial leather with even higher levels of abrasion resistance, dye affinity and surface quality, the fiber layer (A) in the artificial leather of this embodiment is preferably composed of fibers with mean diameters of 1 μm to 8 more preferably 2 μm to 6 μm and even more preferably 2 μm to 5 If the mean diameter of the fibers is 1 μm or greater, the abrasion resistance, dye coloring properties and light fastness will be satisfactory. If the mean diameter of the fibers is 8 μm or smaller, on the other hand, the large number density of the fibers will result in a high luxuriant feel and smooth surface tactile sensation, to obtain artificial leather with more satisfactory surface quality.

The fibers composing the fiber layers of the artificial leather (the fiber layer (A) and the optional fiber layer (B) and additional layers) may be synthetic fibers, which includes polyester-based fibers such as polyethylene terephthalate, polybutylene terephthalate and polytrimethylene terephthalate; and polyamide-based fibers such as nylon 6, nylon 66 and nylon 12. Considering the durability required for use in the field of car seats, for example, polyethylene terephthalate is preferred among these from the viewpoint of excellent color fastness without yellowing of the fibers themselves when exposed to direct sunlight for long periods. From the viewpoint of reducing environmental load, the fibers composing the fiber layer of the artificial leather are more preferably made of chemical-recycled or material-recycled polyethylene terephthalate, or polyethylene terephthalate obtained using a plant-derived starting material.

When it is stated throughout the present specification that the fibers are “dispersed as single fibers”, it means that the fibers do not form fiber bundles similar to the island-component in sea-island composite fibers, for example. Fibers obtained using fibers capable of ultrafine fiber generating such as sea-island composite fibers (with copolymerized polyester as the sea-component and regular polyester as the island-component, for example), with micronizing treatment after three-dimensional entangling with a scrim (removal of the sea-component of the sea-island composite fibers by dissolving decomposition) are present as fiber bundles in the fiber layer (A) and are not dispersed as single fibers. As an example, by preparing sea-island composite cut fibers with an island-component having a single fiber fineness equivalent to 0.2 dtex and 24 island/lf and forming a fiber layer (A) of the sea-island composite cut fibers, and then forming a three-dimensional tangled composite with a scrim by needle punching treatment, filling the three-dimensional tangled composite with a PU resin and dissolving or decomposing the sea-component, it is possible to obtain ultrafine fibers with a single fiber fineness equivalent to 0.2 dtex. In this case the single fibers are present in the fiber layer (A) in a state of fiber bundles with 24 per bundle (equivalent to 4.8 dtex in the bundled state).

When the fiber layer (A) is composed of fibers dispersed as single fibers it is easier to obtain excellent surface smoothness, for example, having uniform naps when the outer surface of the fiber layer (A) has been raised by buffing treatment, and resistance to formation of fiber balls known as “pilling” on the outer surface due to abrasion, even with a relatively low adhesion rate of the PU resin, and therefore artificial leather with more excellent surface quality and abrasion resistance can be obtained. Having the fibers dispersed as single fibers also tends to result in uniform narrow intervals between the fibers, thus providing satisfactory abrasion resistance even though the PU resin is adhering in a micronized form. The method of dispersing the fibers as single fibers may be a method of using a papermaking method to form a fiber sheet of the fibers produced by direct spinning, or a method of dissolving or decomposing the sea-component of a fiber sheet fabricated with sea-island composite fibers to generate ultrafine fiber bundles, and then carrying out the aforementioned water flow dispersion treatment on the ultrafine fiber bundle surfaces to promote formation of single fibers of the ultrafine fiber bundles.

The fibers in a fiber layer other than the fiber layer (A) composing the artificial leather may optionally be or may not be dispersed as single fibers, but according to a preferred aspect, layers other than the fiber layer (A) are also composed of fibers dispersed as single fibers. The fibers composing layers other than the fiber layer (A) are preferably dispersed as single fibers from the viewpoint of forming a uniform thickness of the artificial leather and improving the processing accuracy, and stabilizing the quality, and also from the viewpoint of providing an equal outer appearance on both the front and back of the artificial leather.

If the artificial leather is composed of the fiber layer (A) alone, the basis weight of the fibers of the fiber layer (A) is preferably 40 g/m² to 500 g/m², more preferably 50 g/m² to 370 g/m² and even more preferably 60 g/m² to 320 g/m², from the viewpoint of mechanical strength including abrasion resistance.

When the artificial leather has a three-layer structure of the fiber layer (A), a scrim and the fiber layer (B), the basis weight of the fibers of the fiber layer (A) is preferably 10 g/m² to 200 g/m², more preferably 30 g/m² to 170 g/m² and even more preferably 60 g/m² to 170 g/m², from the viewpoint of mechanical strength including abrasion resistance. The basis weight of the fibers of the fiber layer (B) is preferably 10 g/m² to 200 g/m² and more preferably 20 g/m² to 170 g/m², from the viewpoint of cost and facilitated production. The basis weight of the scrim is preferably 20 g/m² to 150 g/m², more preferably 20 g/m² to 130 g/m² and even more preferably 30 g/m² to 110 g/m² from the viewpoint of mechanical strength and entangling between the fiber layers and scrim.

The basis weight of the artificial leather filled with the PU resin is preferably 50 g/m² to 550 g/m², more preferably 60 g/m² to 400 g/m² and even more preferably 70 g/m² to 350 g/m².

According to one aspect, the texture (stiffness) of the artificial leather is preferably 28 cm or lower, more preferably 6 cm to 26 cm and even more preferably 8 cm to 22 cm. The stiffness is an index representing the texture of the artificial leather. If the stiffness is 28 cm or lower the moldability of the sheet into covering materials or interior finishing materials for interiors, automobiles, aircraft and railway vehicles will be improved and the consumption performance will be satisfactory, while the flexibility will also be more satisfactory for market demand.

According to one aspect, the luxuriant feel (fiber bundle dispersibility) of the artificial leather is preferably grade 4.0 or higher and more preferably grade 5.0 or higher. The luxuriant feel (fiber bundle dispersibility) is a value for assessing compactness of piles on a 7-level scale, based on visual and tactile organoleptic evaluation. A luxuriant feel of grade 4.0 or higher will provide the quality of the sheet as a covering material or interior finishing material for interiors, automobiles, aircraft and railway vehicles.

<Method for Production of Artificial Leather>

An example of a method for producing artificial leather of this embodiment will now be described.

The method of producing artificial leather for this embodiment may include the following steps:

a step of forming a fiber web of sea-island cut fibers, needle punching the web and dissolving the sea-component of the resulting fiber sheet to obtain a fiber sheet with the island-component single fibers exposed; and

a step of subjecting the obtained fiber sheet to water flow dispersion treatment to obtain a fiber sheet with the single fibers dispersed;

and may further include the following steps:

a step of impregnating the fiber sheet with the single fibers dispersed with a water-dispersed PU resin dispersion containing hot water-soluble resin microparticles, and then coagulating the PU resin by heating to obtain a sheet filled with the PU resin; and

a step of using hot water to remove the hot water-soluble resin microparticles from the obtained sheet.

Each step will now be explained in order.

[Step of Forming a Fiber Web of Sea-Island Cut Fibers, Needle Punching the Web and Removing the Sea-Component of the Resulting Fiber Sheet to Obtain a Fiber Sheet with the Island-Component Single Fibers Exposed]

The method of producing each fiber layer forming the fiber sheet of the artificial leather (the fiber layer (A), and the optional fiber layer (B) and other layers) may be a direct spinning method (such as a spunbond method or meltblown method), or a method of forming a fiber sheet with cut fibers (such as a carding method, airlaid method or other dry method, or a wet method such as a papermaking method), which are all suitable, but for this embodiment sea-island fibers (SIF) are used as the starting material. A fiber sheet produced using the cut fibers is preferred because it has low basis weight variation and excellent homogeneity, and tends to form uniform piles and thus improve the surface quality for artificial leather.

Fibers capable of ultrafine fiber generating may be used to form the ultrafine fibers of the fiber sheet. Using fibers capable of ultrafine fiber generating can stabilize the entangling forms of the ultrafine fiber bundles.

The fibers capable of ultrafine fiber generating used may be sea-island fibers having two thermoplastic resin components with different solvent solubilities as the sea-component and island-component, with the island-component as the ultrafine fibers by dissolving removal of the sea-component with a solvent, or peelable composite fibers having two thermoplastic resin components situated alternately in a radial or multilayer manner in the fiber cross-sections and being split into ultrafine fibers by peeled splitting of the components. Sea-island fibers can provide suitable voids between the island-component (ultrafine fibers) by removal of the sea-component, and are therefore preferred for use from the viewpoint of flexibility and texture of the sheet.

Sea-island fibers include sea-island composite fibers obtained by spinning the sea-component and island-component in mutual alignment using a sea-island compositing nozzle, and sea-island mixed fibers obtained by spinning a mixture of the sea-component and island-component. Sea-island composite fibers are preferably used from the viewpoint of obtaining ultrafine fibers of uniform fineness and of obtaining ultrafine fibers of adequate length to contribute to the sheet strength.

The sea-component for sea-island fibers may be a copolymerized polyester obtained by copolymerizing polyethylene, polypropylene, polystyrene, sodiumsulfoisophthalic acid or polyethylene glycol, or polylactic acid. For environmental considerations it is preferred to use an alkali-decomposable copolymerized polyester obtained by copolymerizing sodiumsulfoisophthalic acid or polyethylene glycol, or polylactic acid, which can be decomposed without using organic solvents.

When sea-island fibers are used, the sea-component is preferably removed before the PU resin is applied to the fiber sheet. Removal of the sea-component before application of the PU resin allows the ultrafine fibers to be firmly held since the structure has the PU resin directly adhering to the ultrafine fibers, and therefore results in satisfactory abrasion resistance of the sheet.

The method of entangling the fibers or fiber bundles of the fiber web may be by cutting the sea-island fibers to predetermined fiber lengths to form cut fibers, passing them through a card and cross lapper to form a fiber web, and entangling them by hydroentangling treatment by means of a needle punching or spunlace method.

For a needle punching method, the number of needle barbs used is preferably 1 to 9. If the number of barbs is 1 or greater an entangling effect will be obtained and damage to the fibers can be minimized. If the number of barbs is 9 or less, damage to the fibers can be reduced and fewer needle marks will be left in the artificial leather, allowing the outer appearance of the product to be improved.

Considering the effect on fiber entangling and product appearance, the total barb depth (length from the tip to the base of the barb) is preferably 0.05 mm to 0.10 mm. If the total barb depth is 0.05 mm or greater, the barbs will be able to satisfactorily hook the fibers, allowing efficient fiber entangling to be achieved. If the total barb depth is 0.10 mm or less, fewer needle marks will be left on the artificial leather, resulting in improved quality. Considering the balance between barb strength and fiber entangling, the total barb depth is more preferably 0.06 mm to 0.08 mm.

When the fibers are to be tangled by needle punching, the punch density range is preferably 300/cm² to 6000/cm² and more preferably 1000/cm² to 6000/cm².

The fiber sheet obtained by needle punching may be immersed for 2 minutes in 98° C. water for shrinkage, and dried at a temperature of 100° C. for 5 minutes to form the fiber sheet prior to sea removal.

Removal of the sea-component may be carried out by immersing the sea-island fibers in a solvent to cause constriction. The solvent used to dissolve the sea-component may be an aqueous alkali solution of sodium hydroxide or the like when the sea-component is a copolymerized polyester or polylactic acid. For environment considerations in this step, the removal of the sea-component is preferably carried out with an aqueous alkali solution of sodium hydroxide.

When a method using cut fibers is selected, the cut fiber lengths are preferably 13 mm to 102 mm, more preferably 25 mm to 76 mm and even more preferably 38 mm to 76 mm for a dry method (carding method or airlaid method), and preferably 1 mm to 30 mm, more preferably 2 mm to 25 mm and even more preferably 3 mm to 20 mm for a wet method (papermaking method). For cut fibers used in a wet method (papermaking method), for example, the aspect ratio (L/D), as the ratio of the length (L) and diameter (D), is preferably 500 to 2000 and more preferably 700 to 1500. This aspect ratio range is preferred because the dispersibility and dispersibility of the cut fibers in the slurry of the cut fibers dispersed in water will be satisfactory during preparation of the slurry, the fiber layer strength will be satisfactory, and fiber balls known as “pilling” caused by abrasion will be less likely to be outwardly apparent since the fiber lengths are shorter than by a dry method, allowing the single fibers to more easily disperse. The fiber lengths of cut fibers with diameters of 4 μm, for example, are preferably 2 mm to 8 mm and more preferably 3 mm to 6 mm.

[Step of Subjecting the Obtained Fiber Sheet to Water Flow Dispersion Treatment to Obtain a Fiber Sheet with the Single Fibers Dispersed]

The obtained fiber sheet may be subjected to water flow dispersion treatment to obtain a fiber sheet with the single fibers dispersed. If the water flow dispersion treatment is carried out after the sea-component dissolution step, it will be possible to control the k-nearest neighbor distance ratio (k=9, radius r=20 μm) between cross-sections of the single fibers forming the fiber layer (A) in a cross-section of the artificial leather in the thickness direction, to 80% or lower.

[Step of Impregnating a Water-Dispersed PU Resin Dispersion Containing Hot Water-Soluble Resin Microparticles into the Fiber Sheet with the Single Fibers Dispersed, and then Coagulating the PU Resin by Heating to Obtain a Sheet Filled with the PU Resin]

In this step, the fiber sheet is impregnated with a water-dispersed PU resin dispersion containing hot water-soluble resin microparticles, and then the PU resin is anchored by heating to fill the sheet with the PU resin. As a typical aspect, the PU resin is impregnated as an impregnating liquid in the form of a dispersion (water-dispersed form). The concentration of the PU resin in the impregnating liquid may be 10 to 35 wt %, for example. As another aspect, the impregnating liquid is prepared and impregnated into the fiber sheet so that the ratio of the PU resin is 15 to 50 wt % with respect to 100 wt % of the fiber sheet.

Water-dispersed PU resins are classified as forced-emulsifying PU resins that are forcibly dispersed and stabilized using a surfactant, and self-emulsifying PU resins that have a hydrophilic structure in the PU molecule and disperse and stabilize in water without the presence of a surfactant. While either may be used for this embodiment, forced-emulsifying PU resins are preferably used from the viewpoint of imparting a heat-sensitive coagulating property as explained below.

For this embodiment, the fiber sheet is impregnated with a water-dispersed PU resin dispersion containing hot water-soluble resin microparticles, but it is not preferred for the hot water-soluble resin microparticles to dissolve in the water-dispersed PU resin dispersion. Since the hot water-soluble resin microparticles are less soluble in a water-soluble solution with a dissolved surfactant than in water, a forced-emulsifying PU resin dispersion that contains a surfactant is more preferred than a self-emulsifying PU resin dispersion that does not contain a surfactant. The concentration of the water-dispersed PU resin (the content of the PU resin with respect to the water-dispersed PU resin dispersion) is preferably 10 to 35 wt %, more preferably 15 to 30 wt % and even more preferably 15 to 25 wt %, from the viewpoint of controlling the amount of adhesion of the water-dispersed PU resin and because a high concentration promotes aggregation of the PU resin and reduces the stability of the impregnating liquid.

The water-dispersed PU resin dispersion preferably has a heat-sensitive coagulating property. By using a water-dispersed PU resin dispersion with a heat-sensitive coagulating property it is possible to apply the PU resin evenly in the thickness direction of the fiber sheet. A heat-sensitive coagulating property is a property such that when the PU resin dispersion is heated, the PU resin dispersion loses fluidity and solidifies upon reaching a certain temperature (the heat-sensitive coagulation temperature). After the fiber sheet has been impregnated with the PU resin dispersion during production of the sheet filled with a PU resin, it is coagulated by dry heat coagulation, moist heat coagulation, hot water coagulation or a combination thereof, and dried to apply the PU resin to the fiber sheet. The method currently used in industrial production for coagulating water-dispersed PU resin dispersions that do not exhibit a heat-sensitive coagulating property is dry coagulation, but this tends to cause the migration in which the PU resin becomes concentrated in the surface layer of the sheet, tending to result in hardening of the texture of the sheet filled with the PU resin.

The heat-sensitive coagulation temperature of the water-dispersed PU resin dispersion is preferably 40 to 90° C. If the heat-sensitive coagulation temperature is 40° C. or higher, the storage stability of the PU resin dispersion will be satisfactory and pollution of operating machinery by the PU resin can be inhibited. If the heat-sensitive coagulation temperature is 90° C. or lower, migration of the PU resin in the fiber sheet can be inhibited.

A heat-sensitive coagulant may be added as appropriate to adjust the heat-sensitive coagulation temperature to the range specified above. Examples of heat-sensitive coagulants include inorganic salts such as sodium sulfate, magnesium sulfate, calcium sulfate and calcium chloride, and radical reaction initiators such as sodium persulfate, potassium persulfate, ammonium persulfate, azobisisobutyronitrile and benzoyl peroxide.

The water-dispersed PU resin dispersion may be impregnated and coated onto the fiber sheet, and subjected to dry heat coagulation, moist heat coagulation, hot water coagulation or a combination of these, to coagulate the PU resin. The temperature for moist heat coagulation is preferably 40 to 200° C., and above the heat-sensitive coagulation temperature of the PU resin. If the moist heat coagulation temperature is 40° C. or higher and more preferably 80° C. or higher, then it will be possible to shorten the time until coagulation of the PU resin and better inhibit its migration. If the moist heat coagulation temperature is 200° C. or lower and more preferably 160° C. or lower, heat degradation of the PU resin or PVA resin can be prevented. The temperature for hot water coagulation is preferably 40 to 100° C., and above the heat-sensitive coagulation temperature of the PU resin. If the hot water coagulation temperature in hot water is 40° C. or higher and more preferably 80° C. or higher, then it will be possible to shorten the time until coagulation of the PU resin and better inhibit its migration. The dry coagulation temperature and drying temperature are preferably 80 to 180° C. A dry coagulation temperature and drying temperature of 80° C. or higher and more preferably 90° C. or higher will result in excellent productivity. If the dry coagulation temperature and drying temperature are 180° C. or lower and more preferably 160° C. or lower, heat degradation of the PU resin or PVA resin can be prevented.

As mentioned above, when the fiber sheet with the single fibers dispersed is impregnated with the water-dispersed PU resin dispersion containing the hot water-soluble resin microparticles, the content of the hot water-soluble resin microparticles in the water-dispersed PU resin dispersion is preferably 1 wt % to 20 wt %, more preferably 2 wt % to 15 wt % and even more preferably 3 wt % to 10 wt %. Including hot water-soluble resin microparticles in the water-dispersed PU resin dispersion further promotes dispersion of the PU resin masses.

[Step of Using Hot Water to Remove the Hot Water-Soluble Resin Microparticles from the Obtained Sheet]

The means for removing the hot water-soluble resin from the sheet may be, for example, a method of immersion in hot water at 60° C. or higher and preferably 80° C. or higher, or a method of removing the hot water-soluble resin microparticles while circulating hot water at 80° C. or higher in a jet dyeing machine before dyeing. A method of removing the hot water-soluble resin microparticles in a jet dyeing machine is especially preferred because it can eliminate the steps of drying and winding of the sheet after removal of the hot water-soluble resin microparticles, and thus increase production efficiency. According to this embodiment, removal of the hot water-soluble resin microparticles from the sheet after application of the PU resin yields a flexible sheet. While the method of removing the hot water-soluble resin microparticles is not particularly restricted, a preferred aspect is dissolving removal by immersion of the sheet in hot water at 60 to 100° C., with squeezing using a mangle if necessary.

[Finishing-Process Steps]

After the fiber sheet has been filled with the PU resin and the hot water-soluble resin microparticles have been removed, the sheet filled with the PU resin may be sliced in half horizontally, if it does not include a scrim. This can increase production efficiency.

Before the buffing treatment described below, the PU resin-filled sheet may have a lubricant applied, such as a silicone dispersion. Application of an antistatic agent before buffing treatment is also preferred so that grinding powder generated from the sheet by grinding will be less likely to accumulate on sandpaper.

Buffing treatment may be carried out to form naps on the surface of the sheet. The buffing treatment may be by a method of grinding using sandpaper or a roll sander. Applying silicone as a lubricant before buffing treatment allows naps to be easily formed by surface grinding, resulting in very satisfactory surface quality.

The artificial leather is preferably subjected to dyeing treatment to increase the value for sensibility (i.e. the visual effect). Dyeing may be selected according to the type of fibers composing the fiber sheet, and for example, a disperse dye stuff may be employed for polyester-based fibers, an acid dye or gold-containing dye may be employed for polyamide-based fibers, or any combination of such dyes may be used. When a disperse dye stuff has been used for dyeing, the dyeing may be followed by reduction cleaning. The dyeing method used may be any one commonly known in the dyeing industry. The dyeing method preferably employs a jet dyeing machine to simultaneously provide a rubbing effect while the sheet is dyed, in order to soften the sheet. The dyeing temperature will depend on the type of fiber but is preferably 80 to 150° C. If the dyeing temperature is 80° C. or higher and more preferably 110° C. or higher it will be possible to efficiently dye the fibers. If the dyeing temperature is 150° C. or lower and more preferably 130° C. or lower it will be possible to prevent degradation of the PU resin.

The artificial leather dyed in this manner is preferably subjected to soaping and if necessary reduction cleaning (ore reduction washing) (cleaning in the presence of a chemical reducing agent) to remove the excess dye. It is also preferred to use a dyeing aid during dyeing. Using a dyeing aid can increase the dyeing uniformity and reproducibility. Whether in the same bath as dyeing or after dyeing, finishing process may be carried out using a flexibilizer such as silicone, or an antistatic agent, water-repellent agent, flame retardant, light fastness agent or antimicrobial agent.

The artificial leather of this embodiment can be suitably used as an interior finishing material with a very delicate outer appearance when used as a covering material for furniture, chairs, wall materials, or for seats, ceilings or interior finishings for vehicle interiors of automobiles, electric railcars or aircraft, or as clothing materials used in parts of shirts, jackets, uppers or trims for shoes including casual shoes, sports shoes, men's shoes or women's shoes, or bags, belts and wallets, or even as industrial materials such as wiping cloths, abrasive cloths or CD curtains.

EXAMPLES

The present invention will now be described in greater detail by Examples and Comparative Examples, with the understanding that the invention is not limited by the Examples. The physical properties and quality of the artificial leather samples used in the Examples and Comparative Examples were evaluated by the following protocols and methods.

(1) Sampling Locations

The locations for sampling are shown in FIG. 3.

First, 10 locations at roughly equal distances in the machine direction (MD) of the fiber layer (A) or the artificial leather comprising the fiber layer (A) (sampling regions 1 and 2) are cut out into strips (shown by dotted lines). In each sampling region, a cross-section is formed in the thickness (t) direction. The cross-section formed in the thickness (t) direction is conductively treated by coating with osmium atoms to 1 nm. SEM images are taken at 10 locations at roughly equal distances in the CD perpendicular to the MD, to determine the single fiber cross-sectional k-nearest neighbor distance ratio (%) and cross-sectional PU resin area ratio (%) for the cross-section. To determine the surface PU resin area ratio (%) in each sampling region, the first outer surface of the fiber layer (A) from each of the 10 locations at roughly equal distances in the CD is conductively treated by coating with osmium atoms to 1 nm, and SEM images of the first outer surface are taken. To determine the space size for each sampling region, a three-dimensional images are taken of the 10 locations at roughly equal distances in the CD by X-ray CT. Each image is used to determine the single fiber cross-sectional k-nearest neighbor distance ratio (%), cross-sectional PU resin area ratio (%) and space size, and 100 copies of each image are prepared. Therefore, the average value and the standard deviation are for 100 images.

When the artificial leather has naps, the nap direction is the MD. When the artificial leather does not have naps and the MD is unknown, the sample may be cut out in the direction perpendicular to any arbitrary direction.

(2) Single Fiber Cross-Sectional k-Nearest Neighbor Distance Ratio (%)

As shown in FIG. 5, the k-nearest neighbor algorithm is a method in which k number of single fiber cross-sections near an arbitrary single fiber cross-section are taken and the kth nearest radius by Euclidean distance is used as the decision boundary.

For this embodiment, using one SEM image, a region of ˜250 μm×˜186 μm is photographed with 640×480 pixels including the below image bar (in which case 1 pixel corresponds to ˜0.40 μm×˜0.40 μm), and it is determined whether or not the k=9th nearest single fiber cross-section is present within a distance at a 20 μm radius from the approximate center of the arbitrary single fiber cross-section. This is determined for all of the single fiber cross-sections in a single SEM image, and the single fiber cross-section k=9th nearest neighbor distance ratio (%) is calculated by the following formula:

Single fiber cross-section (k=9th) nearest neighbor distance ratio (%)={(Number of single fiber cross-sections where k=9th nearest single fiber cross-section is present within distance of 20 μm radius from approximate center of single fiber cross-section)/(total number of single fiber cross-sections within one SEM image)}×100.

The k=9th nearest neighbor distance ratio (%) for the single fiber cross-section is the average value of all of the values calculated for the 100 SEM images. When the sample has a scrim, the observation region is the maximum depth of the fiber layer (A) of the cut surface of the conductively treated sample (i.e. the part furthest on the scrim side), and observation is with a scanning electron microscope (SEM, “JSM-5610” by JEOL Corp.), ignoring the fibers forming the scrim. When the sample does not have a scrim, observation is made with the SEM using the center section in the thickness direction of the artificial leather in the cut surface of the conductively treated sample as the center point of the observation region.

The single fiber cross-section in the SEM image may be manually marked for identification, as shown in FIG. 4. The specific procedure is as follows.

[Procedure 1]

After drawing red (R) dots on the fiber cross-sections in the SEM image (gray), the coordinates of the fiber cross-sections are calculated.

<Method in Detail>

(i) OpenCV (cv2 module for Python) is used to read in the image.

(ii) Pixels with R of 220 and G, B 100 (RGB) are extracted.

(iii) For noise processing, the detected dots are passed through dilation processing (cv2.dilate with iteration=2) and erosion processing (cv2.erode with iteration=2).

(iv) The noise treated image is processed by cv2.connected Component With Stats, and the center of gravity coordinates are obtained for the dot detected as the 3rd of the 4 results.

(v) The center of gravity coordinates are used as the location of the fiber cross-section.

(vi) The distances between specific positions on the coordinate system are calculated. Representing the coordinates of fiber cross-section A and fiber cross-section B as (Ax, Ay), (Bx and By), the two Euclidean distances R are calculated as R=√((Ax−Bx)²+(Ay−By)²).

[Procedure 2]

The Euclidean distance to the kth nearest fiber cross-section is calculated for all of the fiber cross-sections (k-nearest neighbor distance: matrix distance).

<Method in Detail>

(i) The distance between the coordinate of the fiber cross-section A and another cross-section is calculated.

(ii) The calculated distances are sorted in ascending order.

(iii) The kth sorted distance is used as the k-nearest neighbor distance.

[Procedure 3]

The number of cross-sections with k-nearest neighbor distance of R or less is divided by the total number of fiber cross-sections, and used as the k-nearest neighbor distance ratio for the SEM image.

For a large number of SEM images, the locations of the fiber cross-sections may be determined with manual marking replaced by machine learning (deep learning) where classification is on the pixel level based on semantic segmentation using the network FCN (Fully Convolutional Networks) method (Jonathan Long, Evan Shelhamer and Trevor Darrel (2015): Fully Convolutional Networks for Semantic Segmentation. In The IEEE Conference on Computer Vision and Pattern Recognition (CVPR)), all consisting of convolution layers, and using an image containing teacher data with red (R) dots marked on the fiber cross-section (correct labels) as the learning data.

(3) Cross-Sectional PU Resin Area Ratio (%) and Standard Deviation

Preprocessing

The sample of the cross-section in the thickness direction is cut to 1 cm×0.5 cm (width (x)×height (y)), and the interior space of the sample is embedded with an epoxy-based resin (base compound: “Queto1812” by Nisshin-EM, curing agent: “MNA” by Nisshin-EM, accelerator: “DMP-30” by Nisshin-EM). The obtained resin-embedded sample is cut parallel to the thickness direction with a microtome to obtain a smooth cut surface. It is then exposed for 4 hours to saturated vapor of ruthenium tetroxide, and the PU resin adhering to the sample is electron stained with ruthenium. It is subsequently coated with osmium atoms to 1 nm for conductive treatment.

Observation

When the sample has a scrim, the observation region is the maximum depth of the fiber layer (A) of the cut surface of the conductively treated sample (i.e. the part furthest on the scrim side), and observation is with a scanning electron microscope (SEM, “SU8220” by Hitachi High-Technologies Corp.), ignoring the fibers forming the scrim. When the sample does not have a scrim, observation is made with the SEM using the center section in the thickness direction of the artificial leather in the cut surface of the conductively treated sample as the center point of the observation region. The observation conditions are as follows.

Acceleration voltage: 10 kV

Detector: YAG-BSE (circular scintillator-type electron backscattering)

Imaging magnification: 500×

Observation field: —230 μm×—173

Image Analysis

The obtained SEM backscattered electron image is binarized by the following method using “ImageJ” image analysis software (version: 1.51j8, National Institutes of Health, USA), and the average size of the PU resin is determined.

(i) The SEM image is subjected to filter processing. The processing conditions are as follows.

Handpass filter processing: Filter large structures down to 40 pixels, Filter small structures up to 3 pixels, Suppress stripes None, Tolerance of direction: 5%, Autoscale after filtering, Saturate image when autoscaling, and for median filter processing, radius: 4, single filtering operation.

(ii) Binarization is by the MaxEntropy method, and the black portions of the SEM image after binarization are defined as PU resin.

(iii) The area ratio of PU resin with respect to each partition is calculated from the binarized image.

As shown in FIG. 6, the obtained binarized image (1280×960 pixels, or 1280×896 pixels excluding the below image bar) is partitioned into 32×32 pixel portions (1120 partitions in this case), and the Analyze Particle function of ImageJ (conditions: Size=0-infinity, Circularity=0.00-1.00) is used to calculate the total area of each PU resin distribution in each partition, recording the value divided by the area of each partition as the cross-sectional PU resin area ratio (%) of each partition. The numbers of pixels are read off in the x and y-axis of the image, the partition size is specified by pixel size, the number of partitions on the x and y-axis are determined, and the PU resin area % in each divided region is calculated.

The area ratio of PU resin calculated from a single SEM image is the average of the PU resin area ratios for all of the partitions in the single SEM image, and the standard deviation is calculated by the formula shown in FIG. 6.

The cross-sectional PU resin area ratio (%) and standard deviation are the average values of the PU resin area ratios and standard deviations calculated from each of the SEM images, for 100 images. As shown in FIG. 6, first the standard deviation for all of the partitions divided in a single SEM image is calculated, and the mean of the standard deviations calculated for each of 100 SEM images is recorded as the standard deviation.

(4) Single Fiber Mean Diameter of Fiber Layer (A) (μm)

The mean diameter of fibers composing the fiber layer (A) is determined by taking 10 SEM images of a cross-section of the fiber layer (A) in the thickness direction forming the artificial leather using a scanning electron microscope (SEM, “JSM-5610” by JEOL Corp.), at a magnification of 1500×, randomly selecting 100 fibers on the first outer surface of the artificial leather, measuring the diameters of the single fiber cross-sections, and determining the arithmetic mean value for the 100 fibers.

When the observed shape of the cross-section of a single fiber was not circular, the distance between the outer circumferences on a straight line perpendicular to the middle point of the longest diameter of the single fiber cross section was taken as the fiber diameter.

FIG. 2 is a conceptual drawing illustrating how a fiber diameter is determined. When the cross-section A of the fiber is elliptical as in FIG. 2, for example, the fiber diameter is considered to be the outer circumferential distance c on a straight line b perpendicular to the midpoint p of the maximum diameter “a” of the cross-section A in the observed image.

(5) Space Size (μm)

The average value in the thickness direction for the diameters (μm) of maximum spheres that fit in the spaces excluding the fibers composing the fiber layer (A) and the PU resin masses, as seen in a three-dimensional image of the fiber layer (A) by X-ray CT, is used as the average space size, determined by the following procedure.

(i) The image is rotated so that the xz axes of the image are within the plane and the y-axis is the thickness direction, and the image is trimmed to a rectangular solid.

(ii) A median filter is applied with the condition of radius 2 pix.

(iii) The Otsu method is used to partition regions. The pixel brightness values are set so that air is 0, and the nonwoven fabric fibers and urethane resin are 255.

(iv) The pixels with brightness values of 255 (fiber, PU resin) are segmented by image processing, and connected structures having a pixel count (pix) of 10,000 for pixels with brightness value of 255 are removed as noise.

(V) Image analysis is performed by the thickness method for brightness values of 0 (air), and the space size is determined. All of the pixels of three-dimensional images have space size values.

(vi) A two-dimensional image is cut out on the xz plane to a thickness of 1 pix along the y-axis (thickness direction), and the mean space size and standard deviation are determined for that plane.

(vii) The procedure of (vi) is repeated for all y, obtaining a profile in the thickness direction.

The space size (μm) and standard deviation are average values calculated for a 100-slice three-dimensional image obtained by X-ray CT. When the sample has a scrim, the observation region is the maximum depth (i.e. the part furthest on the scrim side) of the fiber layer (A), and the image is take with an X-ray CT device (“high-resolution 3D X-ray microscope” by Rigaku Corp.), excluding the fibers of the scrim. When the sample does not have a scrim, the images are taken using the center of the thickness of the cross-section in the thickness direction as the center point of the observation region.

(6) Calculation of Texture (Stiffness)

Each sample was cut to a 20 cm×20 cm square for use as a measuring sample. The measuring sample was set on a horizontal surface, and with the vertices of the square as A, B, C and D, the diagonally opposite vertex A and vertex C were overlaid. Vertex A was set on the horizontal plane and vertex C was laid over vertex A. Next, with vertex C gradually receding from vertex A along the diagonal AC while in contact with the measuring sample, the point where vertex C separated from the measuring sample surface, was designated as point E, and the distance between point E and vertex C was defined as flexible value 1. Flexible value 2 was measured by the same procedure but replacing Vertex A with vertex B and vertex C with vertex D. The arithmetic mean value of flexible value 1 and flexible value 2 was recorded as the texture (stiffness) of the sample. When the artificial leather is a single layer, the average value of 10 samples is used as the texture (stiffness). When the artificial leather has a two-layer or three-layer structure, the average values for 5 samples measured with the fiber layer (A) of the artificial leather facing upward and 5 samples measured with the fiber layer (A) facing downward are used as the texture (stiffness).

(7) Luxuriant Feel (Fiber Bundle Dispersibility)

The samples were visually and organoleptically evaluated by healthy adult males and females (10 each), for a total of 20 evaluators, and evaluated on a 7-level scale with the most frequent evaluation recorded as the luxuriant feel. A luxuriant feel (fiber bundle dispersibility) of grade 4.0 to 7.0 is considered satisfactory (acceptable).

Grade 7: Very dense naps, very satisfactory outer appearance.

Grade 6: Evaluation between grade 7 and grade 5.

Grade 5: Dense naps, satisfactory outer appearance.

Grade 4: Evaluation between grade 5 and grade 3.

Grade 3: Uniform naps overall, leather-like outer appearance.

Grade 2: Evaluation between grade 3 and grade 1.

Grade 1: Mottled naps, poor outer appearance.

The average value for 10 samples was recorded as the luxuriant feel grade.

(8) Adhesion Rate of PU Resin in Fiber Sheet

The adhesion rate of PU resin in the fiber sheet was measured by the following method.

The weight of the fiber sheet before PU resin impregnation is recorded as A (g). The fiber sheet is impregnated with the PU resin dispersion, and then a pin tenter dryer is used for heated air drying at 130° C., after which it is immersed in hot water heated to 90° C. for softening and then dried, to obtain a fiber sheet filled with the PU resin (hereunder also referred to as “resin-filled fiber sheet”). The weight of the resin-filled fiber sheet is designated as B (g). The adhesion rate (C) of the PU resin is calculated by the following formula.

C=(B−A)/A×100 (wt %)

(9) Mean Primary Particle Size of PU Resin in PU Resin Dispersion

Measurement was performed using a laser diffraction particle size distribution analyzer (“LA-920” by Horiba, Ltd.) according to the manufacturer's instruction manual, and the median diameter was recorded as the mean primary particle size.

(10) Saponification Degree of PVA Resin Fine Particles in PU Resin Dispersion

This was measured according to JIS K 6726(1994)3.5.

(11) Polymerization Degree of PVA Resin Fine Particles in PU Resin Dispersion

This was measured according to JIS K 6726(1994)3.7.

(12) Mean Particle Size (Size) of PVA Resin Fine Particles in PU Resin Dispersion (Wn)

The microparticles used may be “NL-05” by Mitsubishi Chemical Holdings Corp., and micronization of the hot water-soluble resin microparticles may be by the method described in Japanese Unexamined Patent Publication HEI No. 7-82384.

(13) Disturbance of Water Stream Discharged from Nozzle During Water Flow Dispersion Treatment

Disturbance of the water stream discharged from the nozzle during water flow dispersion treatment was measured in the following manner.

The water stream discharged from the nozzle is photographed using a single-lens reflex camera (“D600” by Nikon Corp.) equipped with a telecentric lens (“S5LPJ007/212” by Sill Optics GmbH & Co. KG), and the image data is obtained. The image data is loaded into a PC, the range of the water stream is cut off at 25 mm to 35 mm from the discharge hole of the nozzle hole, and the water stream diameter at every pixel row (about 6 μm) in the widthwise direction of the water stream is measured. Based on the total measurement data, the mean diameter W of the water stream and the standard deviation 6 in a region from 25 mm to 35 mm from the discharge hole of the nozzle hole are calculated and the disturbance is calculated by the following formula.

Disturbance (%)=σ (mm)/W (mm)×100

The average value for 5 values obtained from the image data is recorded as the disturbance.

Example 1

Using polyethylene terephthalate copolymerized with 8 mol % sodium 5-sulfoisophthalate as the sea-component and polyethylene terephthalate as the island-component, a sea-island composite fiber with a mean fiber size of 18 μm was obtained with a composite ratio of 20 wt % sea-component and 80 wt % island-component, and 16 islands/lf. The obtained sea-island composite fiber was cut to fiber lengths of 51 mm as cut fibers and passed through a card and cross lapper to form a fiber web, which was then needled-punched to obtain a fiber sheet. The obtained fiber sheet was immersed in 95° C. hot water for contraction and a pin tenter dryer was used for drying at 100° C. for 5 minutes, to obtain a single-layer fiber sheet with a basis weight of 600 g/m².

The obtained fiber sheet was immersed in a 10 g/L sodium hydroxide aqueous solution that had been heated to 95° C. for 25 minutes of treatment, to dissolve the sea-component of the sea-island composite fibers. The single fiber mean diameter of the fibers composing the fiber sheet after sea-component dissolution was 4 μm.

Next, a high-speed water stream was sprayed several times at a pressure of 4 MPa from the upper layer side and 3 MPa from the lower layer side using a 3-row straight flow injection nozzle with a nozzle hole interval of 0.25 mm, a disturbance of 17% and a hole diameter of 0.10 mm, to promote formation of single fibers for the fibers of the fiber bundles.

The fiber sheet was then impregnated with an impregnating liquid comprising the polyether-based water-dispersed PU dispersion “AE-12” (product of Nicca Chemical Co., Ltd.) (solid concentration: 35 wt %) having a mean primary particle size of 0.3 μm in an amount of 9.0% (as solid wt %) in the impregnating liquid, anhydrous sodium sulfate as an auxiliary agent in an amount of 3.0 wt % (as solid wt %) in the impregnating liquid, and the PVA resin fine particles “NL-05” (product of Mitsubishi Chemical Holdings Corp.) with a mean particle diameter of 3 μm, and then moist heat coagulation was carried out at 100° C. for 5 minutes, and a pin tenter dryer was used for hot air drying at 130° C. to 150° C. for 2 to 6 minutes.

It was then immersed in hot water that had been heated to 95° C. to extract and remove the impregnated anhydrous sodium sulfate and PVA resin fine particles, to obtain a sheet filled with the water-dispersed PU resin. The proportion of water-dispersed PU resin with respect to the total weight of the fibers of the sheet was 30 wt %.

A half-cut machine with an endless band knife was then used for half-cutting of the sheet perpendicular to the thickness direction, and the non-half-cut side was subjected to buffing treatment using #400 emery paper, after which a jet dyeing machine was used for dyeing at 130° C. for 15 minutes with a blue disperse dye stuff (“BlueFBL” by Sumitomo Chemical Co., Ltd.) at a 5.0% of dyeing density, and reduction cleaning was carried out. A pin tenter dryer was then used for drying at 100° C. for 5 minutes to obtain single-layer artificial leather.

Example 2

Artificial leather was obtained in the same manner as Example 1, except that the disturbance during the water flow dispersion treatment was changed to 13%.

Example 3

Artificial leather was obtained in the same manner as Example 1, except that the disturbance during the water flow dispersion treatment was changed to 11%.

Example 4

Artificial leather was obtained in the same manner as Example 1, except that the disturbance during the water flow dispersion treatment was changed to 7%.

Example 5

Artificial leather was obtained in the same manner as Example 1, except that the upper layer side pressure of the high-speed water stream was changed to 5.5 MPa.

Example 6

Artificial leather was obtained in the same manner as Example 1, except that the upper layer side pressure of the high-speed water stream was changed to 12.0 MPa.

Example 7

Artificial leather was obtained in the same manner as Example 1, except that the nozzle hole diameter during water flow dispersion treatment was changed to 0.15 mm.

Example 8

Artificial leather was obtained in the same manner as Example 1, except that the nozzle hole diameter during water flow dispersion treatment was changed to 0.22 mm.

Example 9

Artificial leather was obtained in the same manner as Example 1, except that the nozzle hole interval during water flow dispersion treatment was changed to 0.50 mm.

Example 10

Artificial leather was obtained in the same manner as Example 1, except that the nozzle hole interval during water flow dispersion treatment was changed to 0.50 mm, and the number of nozzle hole rows was changed to one.

Example 11

Artificial leather was obtained in the same manner as Example 1, except that the nozzle hole interval during water flow dispersion treatment was changed to 0.90 mm, and the number of nozzle hole rows was changed to one.

Example 12

Artificial leather was obtained in the same manner as Example 1, except that the mean particle diameter of the PVA resin fine particles was changed to 1.5 μm.

Example 13

Artificial leather was obtained in the same manner as Example 1, except that the mean particle diameter of the PVA resin fine particles was changed to 7.0 μm.

Example 14

Artificial leather was obtained in the same manner as Example 1, except that the proportion of PU resin with respect to the fiber sheet was changed to 24 wt %.

Example 15

Artificial leather was obtained in the same manner as Example 1, except that the proportion of PU resin with respect to the fiber sheet was changed to 43 wt %.

Comparative Example 1

Artificial leather was obtained in the same manner as Example 1, except that water flow dispersion treatment was not carried out.

Comparative Example 2

Artificial leather was obtained in the same manner as Example 1, except that PVA resin fine particles were not added to the PU resin impregnating liquid.

Comparative Example 3

Artificial leather was obtained in the same manner as Example 1, except that the mean particle diameter of the PVA resin fine particles was changed to 0.5 μm.

Comparative Example 4

Artificial leather was obtained in the same manner as Example 1, except that the mean particle diameter of the PVA resin fine particles was changed to 11 μm.

Comparative Example 5

Artificial leather was obtained in the same manner as Example 1, except that the proportion of PU resin with respect to the fiber sheet was changed to 14 wt %.

Comparative Example 6

Artificial leather was obtained in the same manner as Example 1, except that the proportion of PU resin with respect to the fiber sheet was changed to 53 wt %.

The results for Examples 1 to 15 and Comparative Examples 1 to 6 are shown in Table 1.

TABLE 1 Conditions Water flow dispersion treatment after sea-component dissolution Water Hole PVA microparticles Carried Disturbance pressure diameter Interval Number Diameter out [%] [MPa] [mm] [mm] of rows Added [μm] Example 1 Yes 17 4 0.1 0.25 3 Yes 3 Example 2 Yes 13 4 0.1 0.25 3 Yes 3 Example 3 Yes 11 4 0.1 0.25 3 Yes 3 Example 4 Yes  7 4 0.1 0.25 3 Yes 3 Example 5 Yes 17 5.5 0.1 0.25 3 Yes 3 Example 6 Yes 17 12 0.1 0.25 3 Yes 3 Example 7 Yes 17 4 0.15 0.25 3 Yes 3 Example 8 Yes 17 4 0.22 0.25 3 Yes 3 Example 9 Yes 17 4 0.1 0.5 2 Yes 3 Example 10 Yes 17 4 0.1 0.5 1 Yes 3 Example 11 Yes 17 4 0.1 0.9 1 Yes 3 Example 12 Yes 17 4 0.1 0.25 3 Yes 1.5 Example 13 Yes 17 4 0.1 0.25 3 Yes 7 Example 14 Yes 17 4 0.1 0.25 3 Yes 3 Example 15 Yes 17 4 0.1 0.25 3 Yes 3 Comp. Example 1 No — — — — — Yes 3 Comp. Example 2 Yes 17 4 0.1 0.25 3 No — Comp. Example 3 Yes 17 4 0.1 0.25 3 Yes 0.5 Comp. Example 4 Yes 17 4 0.1 0.25 3 Yes 11 Comp. Example 5 Yes 17 4 0.1 0.25 3 Yes 3 Comp. Example 6 Yes 17 4 0.1 0.25 3 Yes 3 Results Conditions K-nearest Cross-sectional PU Proportion of neighbor area ratio PU resin in distance Area Standard Space Luxuriant fiber sheet ratio ratio deviation size Stiffness feel [ wt %] [%] [%] [%] [μm] [cm] [grade] Example 1 30 44 21 13 8 18 7 Example 2 30 51 20 13 8 18 7 Example 3 30 53 20 15 9 20 6 Example 4 30 65 19 18 10 21 6 Example 5 30 28 19 11 7 24 6 Example 6 30 17 20 9 6 27 6 Example 7 30 54 21 15 8 17 6 Example 8 30 38 20 11 6 23 6 Example 9 30 61 21 19 14 21 6 Example 10 30 62 20 20 14 21 6 Example 11 30 72 20 22 17 22 4 Example 12 30 46 20 21 18 24 4 Example 13 30 44 21 24 22 25 4 Example 14 24 45 16 20 13 14 4 Example 15 43 45 27 21 7 25 6 Comp. Example 1 30 86 19 19 12 27 3 Comp. Example 2 30 46 21 36 28 27 3 Comp. Example 3 30 46 19 27 23 25 3 Comp. Example 4 30 45 19 33 28 >28 5 Comp. Example 5 14 45 9 23 38 11 2 Comp. Example 6 53 45 33 21 3 >28 3

The results demonstrate that in these Examples, the k-nearest neighbor distance ratio (k=9, radius r=20 μm) between single fiber cross-sections of the fiber layer (A) was 10% to 80%, the cross-sectional PU resin area ratio for cross-sections in the thickness direction was 10% to 30%, and the standard deviation of the cross-sectional PU resin area ratio was 25% or lower, and therefore the PU resin and single fibers were distributed in the specified manner and artificial leather exhibiting texture (stiffness), a luxuriant feel and a slick feel had been obtained.

INDUSTRIAL APPLICABILITY

The artificial leather of the invention has excellent texture (stiffness), and a luxuriant feel and slick feel, and can therefore be satisfactorily used as a sheet covering material or interior finishing material for interiors, automobiles, aircraft or railway vehicles, or in a clothing product. Specifically, the artificial leather of the invention can be suitably used as an interior finishing material with a very delicate outer appearance when used as a covering material for furniture, chairs, wall materials, or for seats, ceilings or interior finishings for vehicle interiors of automobiles, electric railcars or aircraft, or as a clothing material used in parts of shirts, jackets, uppers or trims for shoes including casual shoes, sports shoes, men's shoes or women's shoes, or bags, belts and wallets, or as industrial materials such as wiping cloths, abrasive cloths or CD curtains.

REFERENCE SIGNS LIST

-   1 Fiber sheet -   11 Scrim (optional) -   12 Fiber layer (A) -   13 Fiber layer (B) -   A Cross-section of fiber with elliptical cross-section -   a Maximum diameter of cross-section A -   b Straight line perpendicular to maximum diameter a passing through     midpoint p of maximum diameter a -   c Outer circumferential distance on straight line b -   p Midpoint of maximum diameter a -   MD Machine direction -   CD cross-machine (weft) direction -   t Thickness of artificial leather 

1. Artificial leather comprising a fiber sheet and a polyurethane resin, wherein the fiber sheet contains at least a fiber layer (A) constituting the first outer surface of the artificial leather, the k-nearest neighbor distance ratio (k=9, radius r=20 μm) between cross-sections of the single fibers forming the fiber layer (A) in a cross-section of the artificial leather in the thickness direction is 10% to 80%, the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 10% to 30%, and the standard deviation for the cross-sectional polyurethane resin area ratio in a cross-section of the fiber layer (A) in the thickness direction is 25% or lower.
 2. The artificial leather according to claim 1, wherein in a three-dimensional image obtained by X-ray CT of the fiber layer (A), the average value of space sizes, as the diameters of maximum spheres that fit in the spaces excluding the fibers forming the fiber layer (A) and the polyurethane resin (average space size), is 5 μm to 35 μm in the thickness direction of the fiber layer (A).
 3. The artificial leather according to claim 1, wherein the fiber sheet has a construction of two or more layers comprising a fiber layer (A) constituting the first outer surface, and a scrim and/or fiber layer (B) contacting the fiber layer (A).
 4. The artificial leather according to claim 1, wherein the mean diameter of the single fibers composing the fiber layer (A) is 1.0 μm to 8.0 μm.
 5. The artificial leather according to claim 1, wherein the polyurethane resin is a water-dispersed polyurethane resin.
 6. The artificial leather according to claim 1, wherein the adhesion rate of the polyurethane resin on the fiber sheet is 15 wt % to 50 wt %.
 7. The artificial leather according to claim 1, wherein the stiffness is 28 cm or lower.
 8. The artificial leather according to claim 1, wherein the fiber sheet is composed of polyester fibers.
 9. The artificial leather according to claim 1, wherein the luxuriant feel is grade 4.0 or higher.
 10. A method for producing the artificial leather according to claim 1, which comprises the following steps: a step of forming a fiber web of sea-island cut fibers, needle punching it, and then dissolving the sea-component of the fiber sheet, to obtain a fiber sheet with the island-component single fibers exposed; and a step of subjecting the obtained fiber sheet to water flow dispersion treatment to obtain a fiber sheet with the single fibers dispersed.
 11. The method according to claim 10, which further comprises the following steps: a step of impregnating a water-dispersed polyurethane resin dispersion containing hot water-soluble resin microparticles into the fiber sheet with the single fibers dispersed, and then coagulating the polyurethane resin by heating to obtain a sheet filled with the polyurethane resin, and: a step of using hot water to remove the hot water-soluble resin microparticles from the obtained sheet.
 12. The method according to claim 10 or 11, wherein the hot water-soluble resin microparticles are composed of a polyvinyl alcohol resin.
 13. The method according to claim 10, wherein the water flow dispersion treatment is carried out using a plurality of nozzles having nozzle hole intervals of 1.0 mm or smaller and nozzle hole diameters of 0.05 mm to 0.30 mm.
 14. The method according to claim 10, wherein the water flow dispersion treatment is carried out using a plurality of nozzles that discharge a water stream with a disturbance of 10% or greater.
 15. The method according to claim 11, wherein the solid concentration of the water-dispersed polyurethane resin dispersion is 10 wt % to 35 wt %.
 16. The method according to claim 11, wherein the content of hot water-soluble resin microparticles in the water-dispersed polyurethane resin dispersion is 1 wt % to 20 wt %. 